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        <h1><center>hello,everyone!<br>累了困了，听首歌吧</center></h1>
<iframe frameborder="no" border="0" marginwidth="0" marginheight="0" width=150 height=86 src="//music.163.com/outchain/player?type=2&id=1313052233&auto=1&height=66"></iframe>

<h3 id="将两个表纵向连接-就如同两个集合重叠一样"><a href="#将两个表纵向连接-就如同两个集合重叠一样" class="headerlink" title="% 将两个表纵向连接 就如同两个集合重叠一样"></a>% 将两个表纵向连接 就如同两个集合重叠一样</h3><p>如:<br>select * from userinfo<br>union all<br>select * from userinfo</p>
<h3 id="查看引擎"><a href="#查看引擎" class="headerlink" title="查看引擎"></a>查看引擎</h3><p>show engines<br>InnoDB MyISAM</p>
<h3 id="分页"><a href="#分页" class="headerlink" title="分页"></a>分页</h3><p>limit</p>
<h2 id="1-MySQL查看表结构的SQL命令是什么"><a href="#1-MySQL查看表结构的SQL命令是什么" class="headerlink" title="1.MySQL查看表结构的SQL命令是什么"></a>1.MySQL查看表结构的SQL命令是什么</h2><p>SELECT COLUMN_NAME, COLUMN_TYPE,COLUMN_COMMENT<br>FROM information_schema.COLUMNS<br>WHERE TABLE_NAME = ‘xxx’;</p>
<h2 id="2-Mysql中的分组函数有哪些，功能是什么"><a href="#2-Mysql中的分组函数有哪些，功能是什么" class="headerlink" title="2.Mysql中的分组函数有哪些，功能是什么"></a>2.Mysql中的分组函数有哪些，功能是什么</h2><p>max() 求一组数据的最大值<br>min()求一组数据的最小值<br>sum()求一组数据的总和(只能对数值类型的列或表达式操作)<br>avg()求一组数据的平均值(只能对数值类型的列或表达式操作)<br>count() 求一组数据的记录数<br>统计数/记录数 除了count（*）之外，其他所有分组函数都会忽略列中的空值进行计算</p>
<h2 id="3-结束事务的SQL命令是什么"><a href="#3-结束事务的SQL命令是什么" class="headerlink" title="3.结束事务的SQL命令是什么"></a>3.结束事务的SQL命令是什么</h2><p>commit提交事务<br>rollback回滚事务</p>
<a id="more"></a>

<h2 id="4-使用SQL语句删除表中存在的列"><a href="#4-使用SQL语句删除表中存在的列" class="headerlink" title="4.使用SQL语句删除表中存在的列"></a>4.使用SQL语句删除表中存在的列</h2><p>alter table table_name<br>drop colunm_name</p>
<h2 id="5-查询比SMITH薪资高的员工有哪些"><a href="#5-查询比SMITH薪资高的员工有哪些" class="headerlink" title="5.查询比SMITH薪资高的员工有哪些"></a>5.查询比SMITH薪资高的员工有哪些</h2><p>select *<br>from emp<br>where sal&gt;(select sal from emp where ename=’SMITH’)</p>
<h2 id="6-在视图上可以完成那些操作"><a href="#6-在视图上可以完成那些操作" class="headerlink" title="6.在视图上可以完成那些操作"></a>6.在视图上可以完成那些操作</h2><p>DQL DDL DML<br>在定义视图的子查询中包含分组函数，group by子句和distinct关键字，则不能对视图进行<br>增删改操作。</p>
<h2 id="7-SQL语句中的限制条件用那个关键字表示"><a href="#7-SQL语句中的限制条件用那个关键字表示" class="headerlink" title="7.SQL语句中的限制条件用那个关键字表示"></a>7.SQL语句中的限制条件用那个关键字表示</h2><p>where</p>
<h2 id="8-查询奖金不为空的员工信息"><a href="#8-查询奖金不为空的员工信息" class="headerlink" title="8.查询奖金不为空的员工信息"></a>8.查询奖金不为空的员工信息</h2><p>select *<br>from emp<br>where comm is not null</p>
<h2 id="9-查询每个部门不同工作的最高工资（-）"><a href="#9-查询每个部门不同工作的最高工资（-）" class="headerlink" title="9.查询每个部门不同工作的最高工资（ ）"></a>9.查询每个部门不同工作的最高工资（ ）</h2><p>select max(sal)<br>from emp<br>group by deptno</p>
<h2 id="10-使用SQL命令删除-撤销-数据库中已经存在的表"><a href="#10-使用SQL命令删除-撤销-数据库中已经存在的表" class="headerlink" title="10.使用SQL命令删除(撤销)数据库中已经存在的表"></a>10.使用SQL命令删除(撤销)数据库中已经存在的表</h2><p>drop table 表名</p>
<h2 id="11-将工资低于2000元或者低于平均工资的员工的工资上调20"><a href="#11-将工资低于2000元或者低于平均工资的员工的工资上调20" class="headerlink" title="11.将工资低于2000元或者低于平均工资的员工的工资上调20%"></a>11.将工资低于2000元或者低于平均工资的员工的工资上调20%</h2><p>update table emp<br>set sal=sal*1.2<br>where sal&lt;2000 or sal&lt;(select avg(sal) from emp)</p>
<h2 id="12-返回当前日期的函数是什么"><a href="#12-返回当前日期的函数是什么" class="headerlink" title="12.返回当前日期的函数是什么"></a>12.返回当前日期的函数是什么</h2><p>Now()<br>curtime() curdate()</p>
<h2 id="13-请说明TRUNCATE和DELETE的区别是什么"><a href="#13-请说明TRUNCATE和DELETE的区别是什么" class="headerlink" title="13.请说明TRUNCATE和DELETE的区别是什么"></a>13.请说明TRUNCATE和DELETE的区别是什么</h2><p>TRUNCATE是DDL，只能删除表中所有的，并且释放表存储空间，不能回滚</p>
<p>DELETE是DML，可以删除指定记录，不会释放表存储空间，可以使用ROLLBACK回滚</p>
<h2 id="14-从GROUP-BY分组的结果集中再次进行条件限制的子句是什么"><a href="#14-从GROUP-BY分组的结果集中再次进行条件限制的子句是什么" class="headerlink" title="14.从GROUP BY分组的结果集中再次进行条件限制的子句是什么"></a>14.从GROUP BY分组的结果集中再次进行条件限制的子句是什么</h2><p>Having</p>
<h2 id="15-说明哪种情况下Mysql会自动创建唯一性索引"><a href="#15-说明哪种情况下Mysql会自动创建唯一性索引" class="headerlink" title="15.说明哪种情况下Mysql会自动创建唯一性索引"></a>15.说明哪种情况下Mysql会自动创建唯一性索引</h2><p>对于主键会自动创建唯一性索引(Primary Key)</p>
<h2 id="16-使用SQL命令删除视图"><a href="#16-使用SQL命令删除视图" class="headerlink" title="16.使用SQL命令删除视图"></a>16.使用SQL命令删除视图</h2><p>drop view 视图名</p>
<h2 id="17-查询EMP中名字中有’A’的员工？"><a href="#17-查询EMP中名字中有’A’的员工？" class="headerlink" title="17.查询EMP中名字中有’A’的员工？"></a>17.查询EMP中名字中有’A’的员工？</h2><p>select *<br>from emp<br>where ename like ‘%A%’</p>
<h2 id="18-查询员工姓名和工作地点，没有员工的部门也要显示出来"><a href="#18-查询员工姓名和工作地点，没有员工的部门也要显示出来" class="headerlink" title="18.查询员工姓名和工作地点，没有员工的部门也要显示出来"></a>18.查询员工姓名和工作地点，没有员工的部门也要显示出来</h2><p>select ename,loc,deptno<br>from emp e RIGHT JOIN dept d<br>on e.deptno=d.deptno</p>
<p>19.说明主键约束的作用<br>主键约束是唯一性约束和非空约束的组合，添加了主键约束的列中，<br>不能存在重复数据，也不能为空。主键是每一条记录的标识，保证记录的完整性。</p>
<p>20.有三个表，它们的记录行数分别是10行、2行和6行，<br>三个表进行交叉连接后,会查询出多少条记录<br>交叉连接会产生笛卡尔积，结果为120行记录</p>
<p>21.查询员工编号，薪资和部门号，按照姓名升序排列<br>select empno,sal,deptno<br>from emp<br>order by ename asc</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#null"><span class="nav-number">1.</span> <span class="nav-text">hello,everyone!
累了困了，听首歌吧</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#将两个表纵向连接-就如同两个集合重叠一样"><span class="nav-number">1.0.1.</span> <span class="nav-text">% 将两个表纵向连接 就如同两个集合重叠一样</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#查看引擎"><span class="nav-number">1.0.2.</span> <span class="nav-text">查看引擎</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#分页"><span class="nav-number">1.0.3.</span> <span class="nav-text">分页</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-MySQL查看表结构的SQL命令是什么"><span class="nav-number">1.1.</span> <span class="nav-text">1.MySQL查看表结构的SQL命令是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#2-Mysql中的分组函数有哪些，功能是什么"><span class="nav-number">1.2.</span> <span class="nav-text">2.Mysql中的分组函数有哪些，功能是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#3-结束事务的SQL命令是什么"><span class="nav-number">1.3.</span> <span class="nav-text">3.结束事务的SQL命令是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#4-使用SQL语句删除表中存在的列"><span class="nav-number">1.4.</span> <span class="nav-text">4.使用SQL语句删除表中存在的列</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#5-查询比SMITH薪资高的员工有哪些"><span class="nav-number">1.5.</span> <span class="nav-text">5.查询比SMITH薪资高的员工有哪些</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#6-在视图上可以完成那些操作"><span class="nav-number">1.6.</span> <span class="nav-text">6.在视图上可以完成那些操作</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#7-SQL语句中的限制条件用那个关键字表示"><span class="nav-number">1.7.</span> <span class="nav-text">7.SQL语句中的限制条件用那个关键字表示</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#8-查询奖金不为空的员工信息"><span class="nav-number">1.8.</span> <span class="nav-text">8.查询奖金不为空的员工信息</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#9-查询每个部门不同工作的最高工资（-）"><span class="nav-number">1.9.</span> <span class="nav-text">9.查询每个部门不同工作的最高工资（ ）</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#10-使用SQL命令删除-撤销-数据库中已经存在的表"><span class="nav-number">1.10.</span> <span class="nav-text">10.使用SQL命令删除(撤销)数据库中已经存在的表</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#11-将工资低于2000元或者低于平均工资的员工的工资上调20"><span class="nav-number">1.11.</span> <span class="nav-text">11.将工资低于2000元或者低于平均工资的员工的工资上调20%</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#12-返回当前日期的函数是什么"><span class="nav-number">1.12.</span> <span class="nav-text">12.返回当前日期的函数是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#13-请说明TRUNCATE和DELETE的区别是什么"><span class="nav-number">1.13.</span> <span class="nav-text">13.请说明TRUNCATE和DELETE的区别是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#14-从GROUP-BY分组的结果集中再次进行条件限制的子句是什么"><span class="nav-number">1.14.</span> <span class="nav-text">14.从GROUP BY分组的结果集中再次进行条件限制的子句是什么</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#15-说明哪种情况下Mysql会自动创建唯一性索引"><span class="nav-number">1.15.</span> <span class="nav-text">15.说明哪种情况下Mysql会自动创建唯一性索引</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#16-使用SQL命令删除视图"><span class="nav-number">1.16.</span> <span class="nav-text">16.使用SQL命令删除视图</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#17-查询EMP中名字中有’A’的员工？"><span class="nav-number">1.17.</span> <span class="nav-text">17.查询EMP中名字中有’A’的员工？</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#18-查询员工姓名和工作地点，没有员工的部门也要显示出来"><span class="nav-number">1.18.</span> <span class="nav-text">18.查询员工姓名和工作地点，没有员工的部门也要显示出来</span></a></li></ol></li></ol></div>
            

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